San Juan Dolores Draft Instream Flow Recommendation

Home Browse Search

Attachment 2—Dolores River Instream Flow Recommendation Flow Quantification Methodology PHABSIM and R2Cross Methodology The CPW and BLM evaluated all of the data collected to date and determined that best flow recommendation would be derived from a combination of methods. PHABSIM (Physical Habitat Simulation) is a widely accepted method for quantifying the suitable versus unsuitable hydraulic habitat attributes of selected species and life stages as a function of discharge. R2CROSS is best suited for identifying flows with specific hydraulic criteria across riffle type habitats. The State of Colorado has used R2CROSS extensively in the past to appropriate instream flow water rights. PHABSIM is widely used in North America to quantify instream flow requirements. CPW and BLM determined that exclusive use of the standard R2CROSS method would not be appropriate for this reach of the Dolores River due to its major width(over 100 feet wide in most places), type of fish species present (warm/cool water species) and its big river channel hydraulics that include extensive run, pool, and glide habitat. In addition, only a very small percentage of the fish habitat in this reach is comprised of riffles. For these reasons, CPW and BLM decided to utilize PHABSIM results to develop flow recommendation for the snowmelt runoff months between March and August. This is the portion of the year when the three sensitive species are using run, pool, glide, and riffle habitat to complete important parts of their life cycles, such as spawning and recruitment of young of the year. Habitat availability is critical during this period for maintaining the biomass and age diversity of the three sensitive fish species. CPW and BLM decided to use the R2Cross methodology to develop flow recommendations during the base flow portions of the year from August through March. This is the period when there is substantial competition between individuals for physical habitat space, foraging areas, and limited food supplies. During this period, it is critical for the three species to be able to move between habitat areas to make full use of the limited physical habitat. Riffles are the first location where low flows can limit passage between habitat types, so it as appropriate to develop flow recommendations that focus on the fish passage function. To select an appropriate location for PHABSIM modeling, CPW and BLM staff conducted reconnaissance throughout the 34-mile reach to identify its typical habitat characteristics, including channel widths, substrate types, depths, and velocities. The CPW and BLM staff then selected an 1800-foot reach of stream, located approximately seven miles upstream from the town of Gateway, as a location that could represent the full variety of habitat types found within the 34-mile reach. The CPW and BLM staff established and monumented seven transects that incorporate different mesohabitat types including riffles, runs,pools and glides. These seven different cross-sections formed the basis for the PHABSIM/RHABSIM study conducted by the CPW and BLM.